Various different methods have been published regarding multi-beam systems for optical coherence tomography (“OCT”) that aim at absolute velocity measurement or contrast imaging enhancement of motion occurring inside a sample. In particular, these methods provide a non-invasive way of quantifying blood velocity and improving blood vessel visualizations for biological samples.
First, OCT techniques that provide absolute velocity measurements are typically based on laser Doppler velocimetry. Such techniques typically involve illuminating the sample at a given location, with beams having different incident angles. For example, in the case of spectral domain OCT, one approach is to use polarization multiplexing and a beam displacer to generate two beams, as shown by R. Werkmeister, et al., in “Bidirectional Doppler Fourier-domain optical coherence tomography for measurement of absolute flow velocities in human retinal vessels,” Opt. Lett., Vol. 33, Issue 24 (2008), pp. 2967-2969. Similarly, using a knife edge mirror, two beams can be generated as shown by N. Iftimia et al., in “Dual-beam Fourier domain optical coherence tomography of zebrafish,” Opt. Express, Vol. 16, No. 18 (2008), pp. 13624-13636. These techniques typically require two detectors for acquiring both signals.
Another approach is to encode the two beams with different path lengths, as demonstrated by Pedersen et al. in “Measurement of absolute flow velocity vector using dual-angle, delay-encoded Doppler optical coherence tomography,” Opt. Lett., Vol. 32, No. 5 (2007), pp. 506-508. In this technique a glass plate is positioned midway into the OCT beam path. This technique has the advantage of using a single detector but has the disadvantage of dividing by three the depth range of acquired OCT signal. Additionally, in systems that use only two incident beams, the angle between the measured velocity vector and the plane formed by the two incident beams must be close to zero. If the angle is not close to zero, this method is prone to large velocity measurement errors.
Therefore, other techniques using three beams have been developed, such as, for example, W. Trasischker et al., in “In vitro and in vivo three-dimensional velocity vector measurement by three beam spectral-domain Doppler optical coherence tomography”, J. Biomed. Opt., Vol. 18, No. 11, (2013), pp. 116010-1-116010-11. W. Trasischker et al. utilized three sources and three detectors in the case of spectral domain OCT.
Second, OCT techniques that provide imaging contrast enhancement of motion inside a sample are typically performed with two scanning beams having the same incident angle on the sample. This technique typically involves scanning the sample such that the two beams scan the same location at two different instants. Depending on the delay between the two instants, the motion contrast can be modified. Typically, slow motion is better visualized with a larger delay. Such techniques provide, in living tissues for example, image of blood vessels, namely angiographies.
Various different OCT methods have been published for generating and acquiring the two beams. For example, methods with polarization multiplexing have been demonstrated by Makita et al., in “Dual-beam-scan Doppler optical coherence angiography for birefringence-artifact-free vasculature imaging,” Opt. Express, Vol. 20, No. 3 (2012), pp. 2681-2692 (US Patent US20120120408 A1). In that method, one light source and two detectors were used. However, issues with the sample birefringence may cause contrast deterioration. Another variant uses non-polarizer elements, such as shown by S. Zotter, et al., in “Visualization of microvasculature by dual-beam phase-resolved Doppler optical coherence tomography,” Opt. Express, Vol 19, No. 2 (2011), pp. 1217-1227. But in this case, significant losses of the signal exist. Moreover, the previous method was performed with two sources and two detectors, increasing cost and complexity.
Another approach for generating two beams from a single light source involves encoding each beam with a free-space acousto-optic frequency shifter, as demonstrated by S. Kim et al., “Multi-functional angiographic OFDI using frequency-multiplexed dual-beam illumination”, Opt. Express, Vol. 23, No. 07 (2015), pp. 8939-8947. One disadvantage of this method is the limitation of the distance between beams on the sample due to limited optical bandwidth of the frequency shifter for larger beams.
Therefore, there is a need for a system and/or method for optical coherence tomography that addresses at least some of the problems and disadvantages associated with conventional systems and methods.